I have three rentals on three policies. Is that a problem?
Not necessarily, but it is worth looking at together. The liability limits do not help each other, the renewals drift apart and nobody ever reviews the whole position. Frequently the fix is one umbrella across all of them rather than restructuring each policy.
Does an umbrella cover rental properties?
It can, provided each property is listed and carries the underlying limits the umbrella requires. That requirement is the part people miss: an umbrella sits on top of specified underlying cover and does not float above a property it was never told about.
Should I put the properties in an LLC?
That is a question for your attorney and your accountant rather than your insurance agent, and the answer affects how the policies must be written. If you do, the named insured on each policy has to match the entity that owns the property, which is a detail that gets missed and matters at a claim.